This disclosure relates to the protection of documents. It finds particular application in connection with a method for incorporating data in documents which become visible when the document is copied.
As the quality of color copiers has improved, it has become easier to generate copies of a document which are indistinguishable from the original document. In many instances, the unauthorized copying of document content can have serious implications. For example, there is a concern that color copiers could be used to reproduce security documents, such as checks, stock certificates, automobile title instruments, and other documents of value, for illegal purposes.
One method which is used to authenticate documents and to reduce the unauthorized copying employs what is commonly called the VOID-pantograph. Common techniques for creating pantographs involve forming printed dots, or other elements, of two different sizes and frequencies, which are used to create regions of similar tone, corresponding to a textual warning and background, respectively, in an original (authentic) document. Tone refers to the visual appearance produced by halftone dots, bars, or marks which cover at least a portion of a printed area and which usually have a frequency that is measured in dots, lines, or marks per inch. To provide constant tone, the smaller elements have a higher frequency than the larger elements. Because the tone of the textual warning and the tone of the background pattern are selected to be substantially the same, these two regions have a similar visual impact on an observer of the original document, and the textual warning is not readily perceived.
On copying, however, the situation changes. Since the response of an image sensor employed in the scanner is different from the response of the human visual system, changes in the relative tone of the two different areas will appear. These changes are due to the different frequency response of the sensor with respect to the human eye and also due to other, normally non-linear, effects, such as a detection floor or threshold, where signals below a certain level are simply “lost.” In general, the high frequency components are more strongly affected and attenuated. The difference in response of the scanner expresses itself as a relative change in tone in the copy and thus the hitherto invisible textual warning becomes visible. For example, in the resulting copy, only the larger printed dots are apparent. These larger dots spell out the word “void,” or other pre-determined textual warning.
In current techniques, the pantograph is applied to the substrate to create a pre-printed carrier. An image to be protected is then applied to the pre-printed carrier.
These methods have generally been successful in protecting documents, and are sometimes combined with other techniques, such as the use of camouflage patterns, and the like. However, they are static in nature and thus in general are limited to generally valid, partly nondescript words like “void” or “copy.”
There remains a need for other systems and methods for the creation of pantographs.